Vehicle powertrain systems include an engine that outputs a drive torque to a transmission via a crank shaft. The transmission typically includes a planetary gear arrangement. A plurality of torque transmitting mechanisms in the form of both rotating and stationary clutches and band brakes are adjusted so as to transform the drive torque generated by the engine into multiple forward speed ratios and reverse drive. Conventional powertrain systems typically assign a maximum torque capacity to each of these elements as well as the shafts in the transmission, which when exceeded, may negatively impact or even damage the operation of the powertrain. Thus, conventional powertrain systems operate according to a fixed or static torque-slip value.
Vehicle skid-torque is typically defined as the torque required (i.e., the torque about the axis of the axle shaft, half shaft, etc. of the axle of discussion) to produce a driving force at the point at which the tire is contacting the road surface (i.e., the “contact patch”), that becomes greater than the opposing static frictional force. Overcoming the frictional force (static) will result in a loss of traction. If the frictional forces due to static friction are overcome by the skid-torque, the interaction between the tire and the surface contacting the tire will depend on the kinetic friction characteristics of the two. Generally, the engine and transmission are matched to the fixed skid-torque value such that the torque output by the transmission (i.e., the input torque delivered to the various driveline components) does not exceed maximum skid-torque value regardless of the current vehicle scenario.